The relevant code is using inputname() to try to figure out variable names. inputname() returns empty for any expression giving a value, but returns the variable name in the caller if a plain unindexed variable was passed.
out.newvars.(1) is an expression that pulls out the contents of the first variable of a table, but it is not a plain unindexed variable name, so inputname will return empty.
You should use
out.combinednew = addvars(out.combined,out.newvars.(1), 'NewVariableNames', out.Properties.VariableNames(1));
